The present invention relates to an electromagnetic detector system, and in particular a seismic electromagnetic detector system suitable to detect precursor electromagnetic waveforms of earthquakes.
Often a devastating toll in life and property is taken by earthquakes. It has long been recognized that the toll could be reduced if people within an impending quake's focal area were warned to prepare. Although preparations are unlikely to prevent structural damage to commercial and residential buildings, or to infrastructure such as bridges and roadways, preparation could reduce deaths and serious injuries by people seeking appropriate shelter or retreating from dangerous locations, such as unreinforced brick buildings. Moreover, preparation is likely to reduce the psychological trauma often attributable to an earthquake's sudden onslaught. In addition, preparation will likely reduce personal property causality, such as that related to structural or utility failures, and the fires often associated therewith. Accordingly, it is desirable to forecast the occurrence of earthquakes.
An earthquake's toll results from the seismic waves defining the earthquake. Seismic waves include two types: body waves and surface waves. Body waves comprise primary (or P) waves and secondary (or S) waves that propagate within the earth's body. P waves are longitudinal waves that alternately push (compress) and pull (dilate) the ground in the direction of propagation. S waves are transverse waves that shear the ground in planes perpendicular to the direction of propagation.
Surface waves comprise Love waves and Rayleigh waves that propagate at or near the earth's surface. Love waves shear the ground sideways at right angles to the direction of propagation, much like S waves, but without S waves' vertical shearing. Rayleigh waves displace the earth both vertically and horizontally in a vertical plane that lies in the direction of propagation, whereby a particle of earth will travel an elliptical path as the wave passes, much like a water molecule in rolling ocean waves.
Body waves travel more rapidly than surface waves. Of the surface waves, Love waves generally travel faster than Rayleigh waves and, of the body waves, P waves generally travel faster than S waves. When an earthquake is occurring, the P waves are felt first, like a thud or blow, and thereafter the S waves arrive, as indicated by up-and-down and side-to-side motion. Thereafter, the surface waves strike, causing the ground to shake side-to-side and to roll.
The body and surface waves generally are monitored during an earthquake to gauge the earthquake's intensity. Being contemporaneous with and defining earthquakes, these waves cannot be used for forecasting. Forecasting relies on identification of other physical parameters that, in their occurrence or variance, indicate an impending earthquake. These parameters, when indicating an impending earthquake, are sometimes referred to herein as "precursor seismic activity."
As reported by an article written by Evelyn Roeloffs, a team in 1989 from Stanford University happened to be listening for low-frequency magnetic noise in the Santa Cruz Mountains south of San Francisco with a large coil. On September 12, they noticed an unusual signal with a period between 5 and 20 seconds, which was followed by a background noise increase October 5. On October 17, the background noise rose to a high level, and three hours later the magnitude 7.1 Loma Prieta earthquake hit, rocking the San Francisco Bay area. The earthquake was centered less than five miles from their measurement coil. Since then, one explanation after another for these noise increases have been eliminated, leaving open the possibility that they truly were earthquake precursors. Hoping to repeat the experiment, they have deployed three similar instruments in California, two of them near Parkfield which is a site of intensive earthquake prediction research. Electromagnetic precursors are a controversial subject worldwide. An international group of scientists continues to deliberate whether "seismic electric signals" recorded in Greece are precursors to earthquakes. But there is growing consensus that the Earth's electrical resistance decreased in association with the 1976 Tangshan, China, earthquake. The article goes on to state that as with all potential earthquake prediction techniques, there won't be significant progress until a good-sized earthquake happens in a closely monitored location.
It would appear that the Stanford detector may have detected precursors to the Loma Prieta earthquake so its design has been the basis for further earthquake prediction devices. The Stanford detector is a large coil of wire designed to be resonant in the range of approximately 1--30 hertz. To obtain sensitivity within this range the number of coils and the core for the coils are both selected accordingly. Unfortunately, such a coil is also sensitive to man-made noise, lightning and electromagnetic fields from storms and atmosphere. Accordingly, the Stanford detector needs to be located in remote areas to minimize the detection of extraneous noise. Furthermore, other researchers have had difficulties using the Stanford detector to detect any precursor activity to earthquakes, let alone actually predict earthquakes, so it would appear that the Stanford detector would need to be located near the epicenter of a large earthquake to be effective.
Varotsos et al., U.S. Pat. No. 4,612,506, disclose a method of forecasting earthquakes as a function of transient variations in electric earth current. Varotsos discovered that the electric currents which normally flow in the earth, termed telluric currents, undergo transient changes or variations of a specific nature or character at times before the occurrence of an earthquake. Specifically, Varotsos found that earthquakes are preceded by a first transient variation in telluric earth current measurable as a voltage on the order of hundreds of microvolts per earth-meter having about one minute duration that occurs from six to eight hours before the quake, and a second transient change in earth current measurable as a voltage on the order of tens of millivolts per earth-meter having a duration of a few milliseconds and occurring between thirty seconds and four minutes before the quake. While such a method is useful for prediction of impending earthquakes, the detection of precursor electromagnetic seismic activity only up to eight hours before a quake is an inadequate length of time to warn the public.
Varotsos' detection system involves the simultaneous measurement of pre-earthquake long waves or earth currents at a number of points in the earth by using multiple elongated conductive cylinders. More specifically, the pre-earthquake long waves or earth currents are measured simultaneously at two or more points on the earth surface. Each of the cylinders measures a transitory current that propagates in the crust. The distance that the waves travel in the earth between cylinders results in a small voltage drop of the waves between them. An operational amplifier produces a signal which is the differential potential between the cylinders. The point of origin of the electromagnetic waves or earth currents is computed from the amplitude ratio of the detected signals. As described in Varotsos the cylinders are vertically aligned with axes in orthogonal planes to the direction to the epicenter. Presumably, the theory is that the electromagnetic waves from seismic activity propagate radially outwardly from the epicenter of the pending quake striking the cylinders. With axes in orthogonal planes, the cylinders are oriented to expose the maximum surface area in a direction normal to the epicenter in order to maximize the detected potential difference. Unfortunately, Varotsos' system has noted that periodically earthquakes occur where there were no electromagnetic precursors detected prior to the earthquake.
For the system taught by Varotsos, the changes in earth current preceding an earthquake of a given intensity must be determined empirically for each location because the intensity changes in the earth current are a result of the distance from the earthquake's epicenter, earth conductivity, and the magnitude of the quake itself, all of which vary from location to location. This uncertainty is unacceptable for use in areas that do not experience frequent earthquakes suitable to calibrate the detector. For regions that have infrequent but devastating earthquakes, it would take several disasters to calibrate the detector. In addition, it is difficult to predict earthquakes if the detectors are located on opposing sides of a subterranean feature, such as a fault line.
Tate et al., U.S. Pat. No. 4,628,299 disclose, a seismic warning system using a radio frequency energy monitor. However, such a system is not accurate because changes in the tides and other factors influence the radio frequency field strength which requires statistical calculations for which it attempts to compensate. Accordingly, it is not feasible to discriminate small seismic activity.
Takahashi, U.S. Pat. No. 5,904,943, discloses a system similar to Varotsos et al. that includes a three dimensional distribution of the sources and intensities of the long waves or earth currents, and predicts the focal region, scale and time of occurrence of earthquakes. Takahashi defines the long waves and earth currents as sinusoidal measurements having a frequency not exceeding 300 khz. With the premise that the waves are sinusoidal in nature, Takahashi teaches the use of an antenna for the long waves and measuring the voltage difference between a pair of vertical cylinders for the earth currents. Further, Takahashi teaches that electromagnetic waves over 300 kHz are attenuated at 0.01-1.0 dB/m and therefore seldom observed near the earth surface so no attempt is made to detect waves with higher frequencies. Accordingly, the detectors (cylinders or antennas) are located between 100 km and 500 km from the epicenter depending on the earthquake size to be detected. Takahashi's method includes many of the problems associated with the method taught by Varotsos.
Takahashi proposes that an earthquake is a sudden shifting of the earth's crust along a fault plane that occurs when the stress within the crust comes to exceed the deformation limit. However, as the rock forming the crust of the earth is not homogeneous, smalls-scale disintegration and dislocation of the rock occurs locally at points within the fault plane destined to become the focal region before the earthquake actually occurs. This gives rise to electromagnetic waves (long waves and earth currents). As a result, Takahashi teaches that it becomes possible to predict the occurrence of an earthquake from the electromagnetic waves from the wave source region. However, this method is further limited to about one week prior to the earthquake.
Helms, U.S. Pat. No. 4,507,611, discloses a method of detecting surface and subsurface anomalies of the earth using vertical current measurements. The vertical current manifests itself as alternating current signals which can be measured and represents surface and subsurface anomalies. Local variation in the detected current signals are measured and correlated with the spatial relation to the points of measurement to determine significant measurements indicative of surface and subterranean anomalies.
Weischedel, U.S. Pat. No. 4,219,804, teaches a circuit suitable for identifying electromagnetic radiation signals caused by nuclear detonations and discriminating against false indications by lightning. The circuit is designed to detect the electromagnetic waveform from the nuclear detonation or lightning strike, as a single negative-going waveform. In order to detect the signal an antenna is used. If a signal is detected that exceeds a threshold magnitude, then three circuits are enabled: a zero crossover discriminator, a rise time discriminator, and a precursor discriminator. The zero threshold detector determines whether the signal crosses the zero axis within a required time period, and if this occurs, it provides an output pulse to an "AND" gate. The rise time discriminator determines whether the signal rises to its peak within a required time period, and if this occurs, it provides an output pulse to the "AND" gate. The precursor discriminator compares the peak amplitude detected with a previous signal detected, if any, occurring within a certain previous time. If the previous signal was detected within the prescribed time period then no signal will be provided to the "AND" gate, indicative that this was lightning. If no previous signal was detected within the prescribed time period then a signal is provided to the "AND" gate, indicative of the possibility of a nuclear detonation. The circuit of Weischedel is designed to detect nuclear detonations by sensing a single waveform and discriminate this against a prior waveform within a prescribed time period in order to discriminate against lightning. While suitable for lightning and nuclear detonations, this circuit is not capable of predicting earthquakes.
There are numerous systems for geophysical exploration of the earth primarily for the detection of mineral and oil deposits. These systems are principally based on the detection and analysis of alternating currents, such as generally sinusoidal currents, from within the earth. Some systems impose a waveform into the earth, while other detect changes in existing earth currents. Such systems include, for example; Nilsson, U.S. Pat. No. 3,701,940; Miller, et al. U.S. Pat. No. 4,041,372; T. R. Madden et al., U.S. Pat. No. 3,525,037; Hearn, U.S. Pat. No. 3,976,937; Barringer, U.S. Pat. No. 3,763,419; L. B. Slichter, U.S. Pat. No. 3,136,943; G. H. McLaughlin et al., U.S. Pat. No. 3,126,510; and Weber, U.S. Pat. No. 4,044,299.